You can use Ubuntu Hardy right now. Or Debian Unstable. Or Fedora RawHide. OR whatever. But
most users are not able to cope with the nightly upgrades and weekly breakages.
It takes some debugging to get a stable distibution that will "just work" for a user.
Also: what do you mean by "must be built against the stable release"? A KDE 4 program should
be built against the KDE4 libraries in the stable version or from the nightly build? The libc
version from the stable or from the nightly? Xorg from the stable or from the nightly?